MFN Trainer Column
in Vol. 15 - March Issue - Year 2014
The Latest Trend of The Shot Peening Technology
The main efficiency of shot peening is the fatigue strength improvement due to the compressed residual stress to be given by applying it to strain energy. Various studies and development have been accomplished about this effect regardless of being inside and outside the country, as is known. There are some effects of the shot peening as well. As an example, it is wear-resistance improvement due to the work-hardening with the plastic deformation of the surface and surface region of the materials. For others, tribology properties improve because an irregularity formed on the surface functions as texture. In addition, ratio surface area increases by the surface irregularity formation likewise, and the heat radiation properties are improved as well. Recently, this has attracted attention to improve a friction and wear characteristic (tribological characteristic) using shot peening in Japan. This column introduces them.
The (domestic) Tribology Conference was held recently in Fukuoka, Japan. JAST (Japan Society of Tribologists) hosts this and it is a conference held twice a year, announcing the research and development that a research organization, a university, and a company all based domestically have carried out. Furthermore, it is the biggest conference about tribology, which is sometimes cosponsored with ICMDT (International Conference on Manufacturing, Machine Design and Tribology) or WTC (World Tribology Congress). JAST was established in 1956, and approximately 3,000 researchers of the research organization, a university and the company belong to this as members now. I occasionally act as the editing committee of the journal and participate in the conference mentioned above. This report introduces content announced in the conference as the effect on friction wear characterised by the use of shot peening.
At first, it is the application to a roller rocker arm. The roller rocker arm is one of the valve train system parts in the reciprocating engine, and it catches the power from a cam and takes a role in operating a valve. When it operates, friction occurs to the needle-formed roller bearing of the roller follower. In addition, friction occurs in the parallel aspects to be constructed in the inner wall surface of the rocker arm body. Friction to occur on this parallel surface becomes the dominant factor of power transmission and the mileage. Generally, mirror surface finishing is made on the surface to reduce friction as much as possible. When shot peening is applied to the surface, time required for running is shortened than in the mirror surface state, and a coefficient of friction is reduced earlier. This is because a formed dimple functions as an oil pocket and the decrease in contact area is activated.
Then, when fine particles having lubrication properties are forced into the material surface by shot peening, the coefficient of friction is reduced. In this study, the sliding surface of the roller bearing having a large load to the surface is the target. Unfortunately, the coefficient of friction is not able to decrease more than a mirror surface by the method, but the coefficient of friction shows a stable result from beginning to end in a long-term examination. From this, the finer particles press-fitted by shot peening do not fall off easily, and, depending on the characteristics of the particles, this method illustrates various functions can be given to the materials surface easily.
Moreover, when frictional resistance is reduced using the surface, that quantity of coverage is controlled. Coverage of more than 100% is recommended in the shot peening conventionally; however, quantity of coverage is controlled in this study because a developed shot peening device is used. It is a key point succeeding in control of the quantity of coverage in fine particle peening having difficulty in control to be characteristic. Even if fine particles of 50 microns are used for projection materials, they form a surface with less than 20% of coverage. The processed surface shows not only the low frictional coefficient but also longer life than a mirror surface. When dimples are placed thinly, the left surface can catch the large contact pressure, and the dimples function as an oil pocket and storage space of wear particles.
In this way, shot peening is evolving every day, and, as for the film formation technology to the materials surface by the particle projection under low temperature environment called the cold spray, research and development are done in another field. "Shot peening = fatigue strength improvement" is natural, but it was at the conference that I keenly realized I could contribute to various fields using shot peening still more even now.
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Author: Masafumi Ando